Conventionally, an ECM (Electret Condenser Microphone) has been widely used as a small-sized microphone that is mounted on a cellular phone or the like. However, the ECM is weak against heat, and a MEMS microphone is superior in terms of coping with digitalization, of miniaturization, of enhancement of functionality/multi-functionality, and of power saving. Accordingly, at present, the MEMS microphone is becoming widespread.
The MEMS microphone includes an acoustic sensor (acoustic transducer) that detects an acoustic wave, and an output IC (Integrated Circuit) that amplifies a detection signal from the acoustic sensor and outputs the detection signal thus amplified to outside. This acoustic sensor is manufactured by using the MEMS technique (for example, Patent Literature 1 and the like).
FIG. 8 schematically shows a configuration of a conventional acoustic sensor. (a) of FIG. 8 is a plan view, and (b) of FIG. 8 is a cross-sectional view taken along the line X-X of (a) of FIG. 8 as viewed in the direction of the arrows. As shown in FIG. 8, an acoustic sensor 111 includes: a semiconductor substrate 21; a vibrating membrane 22 provided above the semiconductor substrate 21; and a fixed membrane 123 provided so as to cover the vibrating membrane 22. The vibrating membrane 22 is a conductor, and functions as a vibrating electrode 22a. Meanwhile, the fixed membrane 123 includes: a fixed electrode 123a, which serves as a conductor; and a protecting membrane 123b, which serves as an insulator for protecting the fixed electrode 123a. The vibrating electrode 22a and the fixed electrode 123a are opposed to each other with a gap sandwiched therebetween, and function as a capacitor.
The vibrating membrane 22 has an edge portion attached to the semiconductor substrate 21 with an insulating layer 30 sandwiched therebetween. Moreover, the semiconductor substrate 21 has an opening 31 made by opening a region opposed to a central part of the vibrating membrane 22. Furthermore, the fixed membrane 123 has a large number of sound hole portions 32 in which sound holes are formed. Normally, the sound hole portions 32 are regularly arrayed at equal intervals, and the sound holes in their respective sound hole portions 32 are of substantially equal in size to one another.
In the acoustic sensor 111 thus configured, the acoustic wave from the outside reaches the vibrating membrane 22 through the sound hole portions 32 of the fixed membrane 123. At this time, since the application of a sound pressure of the reached acoustic wave causes the vibrating membrane 22 to vibrate, the distance between the vibrating electrode 22a and the fixed electrode 123a changes, so that the capacitance between the vibrating electrode 22a and the fixed electrode 123a changes. By converting such a change in capacitance into a change in voltage or in current, the acoustic sensor 111 can detect the acoustic wave from the outside and convert the detected acoustic wave into an electrical signal (detection signal).
The acoustic sensor 111 thus configured has the large number of sound hole portions 32 in the fixed membrane 123. Besides allowing the acoustic wave from the outside to pass therethrough and to reach the vibrating membrane 22, the sound hole portions 32 function as follows:
(1) The acoustic wave that has reached the fixed membrane 123 passes through the sound hole portions 32, and accordingly, the sound pressure to be applied to the fixed membrane 123 is reduced.
(2) Air between the vibrating membrane 22 and the fixed membrane 123 goes in and out through the sound hole portions 32, and accordingly, thermal noise (air fluctuations) is reduced. Moreover, damping of the vibrating membrane 22, which is caused by the air, is reduced, and accordingly, a deterioration in high-frequency characteristics by the damping is reduced.
(3) The sound hole portions 32 can be used as etching holes in the case of formation of the gap between the vibrating electrode 22a and the fixed electrode 123a by use of a surface micromachining technique.